1. Field of the Invention
This invention relates broadly to monitoring systems. More specifically, this invention relates to a system and method for monitoring a dynamic system, such as a boiler or heat exchanger, which tends to undergo operational fluctuations, and for detecting quickly and accurately the occurrence of an event, such as a leak or lost efficiency, which may otherwise be masked or falsely implied by the operational fluctuations.
2. Background
The early detection of an event such as a leak or malfunction can be critical in many applications. As used herein, the term xe2x80x9ceventxe2x80x9d broadly refers to an occurrence which impacts the normal operational of a particular system, and which, in many cases, tends to grow worse or more intense if left unattended. For example, in the case of black liquor recovery boilers used in the paper industry, detecting a leak in the boiler tubes early often is the difference between a controlled shut down and a catastrophic explosion. Such explosions not only result in the destruction of the boiler, but also frequently in severe injury and death.
Detecting the occurrence of a real and significant event in a system, particularly a complex system like a black liquor recovery boiler, poses a challenging task. Often such systems have many variables and their operation is subject to considerable, yet normal, changes and fluctuations. These normal operational changes and fluctuations tend to mask events or create the false appear of one. Therefore, a monitoring system for detecting the occurrence of an event must be able to filter out these normal fluctuations, but nevertheless remain sensitive enough to provide an early indication of an event.
In the case of recovery boilers, methods to detect water leaks have been proposed and tested by researchers for several years. Most leak detection schemes monitor the total airborne or structural acoustic energy levels within the boiler to determine if a leak is present. Such acoustic leak detection strategies, however, face significant limitations. These limitations include the interference presented by high background noise levels, the attenuation of acoustic energy occurring within the boiler, and the limited durability of acoustic sensors in the harsh environment of the boiler.
A more recent improvement of acoustic leak detection systems, developed by Westvaco Corporation, utilizes spectral analysis techniques (TAPPI Journal (July 1990)). While the Westvaco system appears to be an improvement over previous acoustic leak detection systems, it still, to some extent, is subject to the above-identified disadvantages of acoustic leak detection technology. Moreover, spectral-analysis, acoustic leak detection systems tend to be expensive to purchase, implement and maintain.
A difference approach to leak detection system, which was introduced by the Applicant in U.S. Pat. No. 5,363,693, detects a leak by striking a mass balance around the recovery boiler. By measuring the fluid flowing in to and out of the recovery boiler system, fluid leakage loss based on the difference between the input and output can be calculated. If the difference is determined to be significant, a leak is indicated.
Although this leak detection system represents a marked improvement over existing acoustic methods, an on-going need exists to improve the sensitivity and reliability of a leak detection system. Moreover, the Applicant perceives a general need for a system that can quickly and accurately detect an occurrence of an event or significant change in all types of dynamic systems which undergo normal fluctuations. The present invention fulfills these needs among others.
The present invention provides for the early and accurate detection of an event or significant change in a target system by monitoring and statically evaluating at least one operational variable of the target system. The operational variable may be related to mass, volume, energy, momentum, time, money, quality, or any other quantitative measurement or calculation that provides an indication of one or more conditions of the target system which change in response to the event. The operational variable is evaluated by periodically averaging its values over relatively short and long terms. By comparing the long and short term averages using convenient, readily-implemented statistical techniques, the occurrence of an event can be determined with a high degree of confidence despite the target system""s normal fluctuations.
One aspect of the present invention is a method for detecting an occurrence of an event in a target system. In a preferred embodiment, the method comprises the steps of: (a) calculating short and long term averages of sequentially recorded values of an operational variable wherein the long term average includes at least one values in addition to those included in the short term average; (b) calculating a difference, xcex94, between the short and long term averages; and (c) statistically evaluating whether xcex94 is significant and indicative of an event occurrence.
In one embodiment, the statistical evaluation of step (c) comprises an analysis of xcex94 to determine its significance given the number of values considered in the averages and the variation between the values. The reliability of the indication is increased by repeatedly calculating whether a significant xcex94 exists. As the number of sequential significant xcex94s increases, so does the certainty that an event has occurred.
A preferred technique for determining the significance of A comprises calculating ttest. This calculation provides an important correlation between the sampling size and the certainty that a xcex94 is real as opposed to just a random anomaly or xe2x80x9cnoise.xe2x80x9d
In another embodiment, the statistical evaluation of step (c) comprises an analysis of the change in a over time. By analyzing the dynamics of xcex94, a trend indicative of the event can be determined well before the event""s full effect is realized by static measurements. In other words, the present invention anticipates an event based upon the trend of xcex94s, rather than waiting until xcex94 reaches a predetermined limit. This approach not only shortens the time in which an event can be determined over prior art methods, but also improves the accuracy of such determinations and reduces the occurrence of false positive alarms.
The dynamic analysis mentioned above may be performed by repeating steps (a) and (b) at least once, wherein the short and long term averages include at least one value more recent than any values considered in a previously calculated short and long term average, and then calculating the change in xcex94 over the change in sequence of the recorded values. If the change in xcex94 exceeds a predetermined value, then the occurrence of an event is likely.
Another aspect of the present invention is a system for detecting an occurrence of an event in a target system. In a preferred embodiment, the system comprises a central processing unit (CPU), user interface means operatively connected to the CPU for inputing the target system""s operational variable(s) and for outputing an indication of an occurrence of an event, and memory which contains a program of instructional means for the system to perform the above-mentioned process. The system may also comprise measurement means for measuring the target system""s inputs and outputs.
Still another aspect of the present invention is a computer readable medium, such as a floppy disk or CD ROM. The computer readable medium contains a program of instructional means for the above-mentioned system to perform the above-mentioned process. In accordance with the present invention, the method and system provided enable a user to monitor a dynamic system which tends to undergo operational fluctuations, and to detect quickly and accurately the occurrence of a significant event which may otherwise be masked or falsely implied by the operational fluctuations. Applications for the present invention are widespread and include, for example, monitoring a mass balance around a boiler to detect a leak; monitoring the heat transfer of a heat exchanger to detect excess scale build up; monitoring a motor""s power consumption to detect bearing failure; monitoring overtime to detect an under-staffing condition; monitoring service calls to detect a defective product; and monitoring quality on an assembly line to detect low moral. Indeed, the present invention can be applied to any target system or situation having at least one operation variable that changes in response to an event.